This invention relates generally to an electrochromic display device, and more particularly to an electrochromic display device having a simplified driving circuit.
Electrochromic devices are well-known devices which exhibit a phenomenon known as "persistent electrochromism", e.g. see U.S. Pat. No. 3,521,941 entitled, "Electro-Optical Device Having Variable Optical Density", issued July 28, 1970. The term "persistent electrochromism" denotes the property of a material whereby its electromagnetic radiation absorption characteristic is altered, in most instances, even at ambient temperature, under the influence of an electric field. Such materials, for example, may exhibit little or no absorption of visible wavelength in the absence of an electric field and therefore be transparent, but when subjected to an electric field, effectively absorb in the red end of the spectrum, thereby turning blue in color. Similar effects can be observed in other portions of the electromagnetic spectrum, invisible as well as visible.
There have been numerous studies of the electrochromic process which occurs at the electrochromic electrodes. Various attempts at application of the electrochromic devices, such as in numerical displays, have been made, but most of those attempts have been discouraged by defects of the electrochromic devices, e.g. slow coloring and bleaching response, short cycle lifetime of coloring and bleaching, and problems with the method driving an electrochromic display device.
As described in earlier-filed U.S. patent application Ser. No. 953,596 filed Oct. 23, 1978, the electrochromic layers deposited by oblique evaporation have fast coloring and bleaching responses. This improvement made it possible to replace the H.sub.2 SO.sub.4 electrolyte by LiClO.sub.4 and extended the cycle lifetime. But the problems encountered with the driving of such an electrochromic display device have not been solved. Because the coloring and bleaching responses depend on the area of the electrochromic layers and the ambient temperature, the optical density of the colored electrochromic layers differs. For example, in case of displaying a figure "1", "7" or the like which has a small display area, the color of the display turns deep immediately, however, in case of displaying "8", "9", "6" or the like which has a large area, the color does not turn deep immediately, so that the shade of color of one figure may differ from that of another figure. It is very complicated and difficult to control the driving condition and to make an integrated circuit.